1. Field of the Invention
The present invention relates to a surface position detecting apparatus and a method therefor. More particularly, the invention relates to an apparatus and a method for detecting a surface position, which are suitably applicable to detection of surface position (height distribution) of a semiconductor wafer on which a circuit pattern with directionality is formed, for example in projection exposure apparatus (as so called as steppers) for fabricating semiconductors.
2. Related Background Art
Exposure apparatus are used to transfer a circuit pattern formed on a photomask or reticle (hereinafter generally referred to as "reticle") onto a photosensitive-material-coated wafer through a projection optical system. Such an exposure apparatus is provided with an autofocus mechanism and an auto-leveling mechanism for effecting exposure under the condition that an exposed surface of wafer is aligned with an image-forming plane of projection optical system within the range of depth of focus. The autofocus mechanism is for setting each of positions (focus positions) of predetermined measurement points on the exposed surface of wafer in the direction of the optical axis of projection optical system, at the best focus position, while the auto-leveling mechanism is for setting an average plane of the exposed surface of wafer in parallel with the image-forming plane of projection optical system.
For accurate operations of the autofocus mechanism or the auto-leveling mechanism, it is necessary for a surface position detecting apparatus to accurately measure a focus position of a selected one or each of plural measurement points on the exposed surface of wafer. The projection optical system of exposure apparatus has a relatively shallow depth of focus and there is a case that recesses and projections are locally scattered on a wafer. Therefore, the surface position detecting apparatus is required to accurately detect a focus position of each selected measurement point on the exposed surface of wafer.
A conventionally known surface position detecting apparatus is for example an oblique incidence autofocus sensor (hereinafter referred to as "AF sensor") which projects a slit pattern image in an oblique direction onto measurement points on an exposed surface or detected surface of wafer and which collects reflected light from the detected surface to re-image the slit pattern image. In this case, a vertical movement of the exposed surface of wafer in the normal direction (the direction of the optical axis of projection optical system) causes a horizontal shift of the re-imaged position of slit pattern image. Then a focus signal corresponding to the focus position of each measurement point on the exposed surface can be obtained for example by vibrating the slit pattern image on a light-receiving element using a vibrating mirror and then performing synchronous rectification of an output signal from the light-receiving element with a drive signal of the vibrating mirror.
An example of conventional oblique incidence AF sensor is one as disclosed in Japanese Patent Application No. 4-247748, in which a slit pattern image is projected onto numerous two-dimensionally distributed measurement points on a detected surface, the position detection is carried out by the vibration method, and the light reception efficiency is enhanced using a tilt correcting prism. Also, there is another example as disclosed in Japanese Patent Application No. 2-401880, in which a slit pattern image is projected onto numerous one-dimensionally distributed measurement points on a diagonal line of shot area (chip pattern) on the exposed surface of wafer and the position detection is carried out by the vibration method.
Further, Japanese Patent Application No. 3-311758 discloses a position detecting apparatus of an image processing method, in which a two-dimensional pattern is obliquely projected onto an exposed surface of wafer and a re-focused image of the pattern is subjected to image processing whereby focus positions are detected for numerous measurement points on the exposed surface. This apparatus also employs a tilt correcting prism to enhance the light reception efficiency. Japanese Patent Publication No. 1-41962 discloses another apparatus, in which a slit pattern image is projected onto a measurement point on a detected surface and the direction of slit pattern image is inclined relative to the direction of circuit pattern on the detected surface.
Generally, in case of the detected surface being an exposed surface of semiconductor wafer, an IC chip pattern is formed on the exposed surface in preceding steps. Further, a contour of IC chip pattern is usually rectangular and the chip pattern is often parallel to the sides of the rectangular contour. Then, in order to reduce influence of the chip pattern on the exposed surface of wafer, the conventional surface position detecting apparatus are so arranged that a measurement pattern image is projected onto the exposed surface of wafer with the longitudinal direction of each slit in the measurement slit pattern image making an angle of about 45.degree. with each side of the contour of chip pattern.
Taking a reference plane on a plane formed by the optical axis of a light-supply optical system or irradiation optical system in the surface position detecting apparatus and the normal line to the exposed surface, as the exposed surface is displaced in the direction of the optical axis of projection optical system, the measurement pattern image is shifted in parallel with the reference plane. The conventional apparatus are thus so arranged in order to maximize the detection sensitivity that the reference plane becomes perpendicular to the longitudinal direction of each slit in the measurement pattern image. However, in case the position detection is carried out for multiple points on the exposed surface of wafer while keeping the reference plane perpendicular to the longitudinal direction of measurement pattern image as described, the light-supply optical system and light-reception optical system in the surface position detecting apparatus need to cover an area with a width greater than the diagonal line of the contour of chip pattern, which caused a disadvantage of a size increase of optical systems. This results in an increase in production cost of apparatus and an increase of unstable factors in measurement results due to the size increase of apparatus.
Further, in case the reference plane is perpendicular to the longitudinal direction of measurement pattern as in the conventional apparatus, the chip pattern defines the direction of incidence of light for position detection, which is the direction of reference plane. This causes a problem that the arrangement of optical systems in the surface position detecting apparatus can not be changed for example as to match the constitution of semiconductor exposure apparatus.